Oil return in refrigerant system

ABSTRACT

To address the problem of lubricant entrainment within the refrigerant system components such as an evaporator and suction line, a control is provided to periodically, substantially and intermittently increase the refrigerant flow through these components to thereby carry the trapped lubricant back to the compressor. The increased flow of refrigerant can be accomplished by periodically throttling and then unthrottling either an expansion device or a suction modulation valve to cause instantaneous pressure buildup within a respective section of the vapor compression system and subsequent increase of the refrigerant flow through the above-referenced components such as an evaporator and suction line. Suggested time intervals of both the throttling and unthrottling states are provided, as well as the frequency of occurrence for subsequent oil return cycles.

BACKGROUND OF THE INVENTION

This invention relates generally to air conditioning and refrigerationsystems and, more particularly, to a method of oil return to arefrigerant compressor to ensure adequate lubrication of the compressorcomponents and with minimal or no performance degradation of arefrigerant system.

In a vapor compression system such as that used in air conditioners,heat pumps and refrigeration units, refrigerant vapor from an evaporatoris drawn in by a compressor, which then delivers the compressedrefrigerant to a condenser (or a gas cooler for transcriticalapplications). In the condenser, heat is exchanged between a secondaryfluid such as air or water and the refrigerant, and from the condenser,the refrigerant, typically in a liquid state, passes to an expansiondevice, where the refrigerant is expanded to a lower pressure andtemperature, and then passes to the evaporator. In the air conditioningapplications, in the evaporator, heat is exchanged between therefrigerant and another secondary fluid such the indoor air or water tocondition the indoor air or to cool water.

Since the refrigerant compressor necessarily involves moving parts, itis typically required to provide lubrication to these parts by means oflubricating oil that is mixed with or entrained in the refrigerantpassing through the compressor. Although the lubricant is normally notuseful within the system other than in the compressor, its presence inthe system does not generally detract from the flow and change of stateas the refrigerant passes through the system in a conventional vaporcompression cycle. However, there is a tendency for oil to be retainedwithin the evaporator or suction line of the refrigerant system. This isparticularly true in a system wherein the evaporator is of amicrochannel heat exchanger type and when refrigerant mass flow ratesare low. If the oil retention in the evaporator becomes excessive, thenthe performance of the evaporator, as well as that of the entire system,is degraded due to heat transfer reduction and pressure drop increase.More importantly, the oil retention in the evaporator or suction linemay reduce the amount of lubricant passing through the compressor suchthat it is not adequately lubricated, and damage may occur to thecompressor components. In the most severe scenario, all oil can bepumped out of the compressor, leaving the compressor internal elementsessentially with no lubrication and leading to quick seizure of thecompressor.

One approach to solving this problem is that of providing an oilseparator downstream of the compressor such that the oil is removed fromthe refrigerant prior to passing through the remaining sections of thesystem. However, an oil separator represents an added expense that isnot desirable. Further, oil separators are never 100% efficient, sosooner or later a significant amount of oil may be trapped in therefrigerant system components (other than a compressor) causingabovementioned problems. Oil separators can malfunction (plug up, springa leak, etc.), would often introduce additional undesirable pressurelosses and have an inherent high-to-low pressure refrigerant leak sincethe oil needs to be returned from a high pressure discharge section backto a low pressure side (normally, a compressor oil sump). Therefore,there is a need for a cost effective method to assure oil return to thecompressor that preferably doesn't require any extra components added toa refrigerant system.

SUMMARY OF THE INVENTION

Briefly, in accordance with one aspect of the invention, the amount ofrefrigerant flowing through the evaporator is periodically, suddenly andsubstantially increased such that the higher mass flow of refrigerantwill carry the oil trapped in the evaporator and suction line back tothe compressor.

By yet another aspect of the invention, the increase in refrigerant flowthrough the evaporator can be accomplished by throttling/unthrottlingthe expansion device to provide a blast of high pressure refrigerantthrough the evaporator.

By yet another aspect of the invention, the increase in refrigerant flowthrough the evaporator can be accomplished by throttling/unthrottlingthe suction modulation valve between the evaporator and the compressorto provide a blast of refrigerant through the evaporator.

In the drawings as hereinafter described, a preferred embodiment isdepicted; however, various other modifications and alternateconstructions can be made thereto without departing from the spirit andscope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a refrigerant system with a control that operates inaccordance with the present invention.

FIG. 1 b is a graphic illustration of the compressor discharge pressureas a function of time in accordance with the present invention.

FIG. 2 a is a schematic illustration of an alternative embodiment of theinvention.

FIG. 2 b is a graphic illustration of the compressor suction pressure asa function of time in accordance with the alternative embodiment of theinvention.

FIG. 2 c is a graphic illustration of the refrigerant mass flow ratethrough the evaporator when at least one of the devices (the electronicexpansion device or the suction modulation valve) isthrottled/unthrottled.

FIG. 2 d is a graphic illustration of the refrigerant mass flow ratethrough the evaporator when at least one of the electronic expansiondevice or suction modulation valve is widely opened for a relativelyshort period of time.

FIG. 3 is a flow chart illustrating a method in accordance with oneembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is intended for use in a vapor compression system10, which includes in serial flow relationship a compressor 11, acondenser 12, an expansion device 13 and an evaporator 14. Thecompressor 11, which requires a certain amount of lubricant to properlylubricate its internal moving components, compresses the refrigerantvapor having lubricant entrained therein and passes it on to thecondenser 12 where the refrigerant is condensed to a liquid. The liquidrefrigerant and lubricant mixture passes to the expansion device 13,where some of the liquid refrigerant flashes to a vapor, and a two-phaserefrigerant mixture then passes, along with the liquid lubricant, to theevaporator 14 from which it is returned to the compressor 11 to completethe cycle. It has to be noted that although a very basic refrigerantsystem configuration is described above, many additional options andfeatures are feasible, and the corresponding refrigerant systemschematics will be within the scope of the invention.

Although oil can be trapped in various locations within the refrigerantsystem 10, the evaporator 14 typically has a higher tendency to entraina certain amount of lubricant within its volume. This is particular truein the case where an evaporator construction is of a microchannel heatexchanger type, which has a plurality of small passages within each heattransfer tube, and at low refrigerant flows, which are typical forpart-load conditions or low temperature refrigeration applications.Additionally, increased oil viscosity at low temperatures, as well aspotential miscibility and solubility issues, aggravate the problem inhand. If the accumulation of lubricant in the evaporator 14 becomesexcessive, there will not be a sufficient amount of lubricant gettingback to the compressor 11, and the compressor component frictionaloverheating results in nuisance shutdowns and/or subsequent permanentdamage to the compressor. Also, with the accumulation of lubricant inthe evaporator 14, the refrigerant in-tube thermal and hydraulicresistances will increase negatively affecting the evaporator and entiresystem performance. Furthermore, in certain types of compressors, suchas scrolls and screws, oil is relied upon to seal the gaps between thecompression elements to prevent refrigerant leakage from high to lowpressure compression chambers. Therefore, an insufficient amount of oilwill reduce compressor volumetric and isentropic efficiencies and theamount of refrigerant delivered throughout the refrigerant system 10.

The expansion device 13 is an electronically controlled expansion valvewith a variable orifice for selectively varying the amount ofrefrigerant that is allowed to pass therethrough and to the evaporator14 as a vapor and liquid mixture. Typically, the expansion valve 13 isactivated and controlled by a stepper motor (not shown) utilizing sensorfeedback of the evaporator superheat to a system control 17. Suchsensors can be temperature and/or pressure transducers. These sensorsare typically positioned at the suction line locations between theevaporator 14 and compressor 11 (usually at the evaporator outlet) andprovide measurements of the evaporator superheat to the systemcontroller 17. This allows the valve to be operated in the manner so asto maintain a consistent superheat at the evaporator outlet, regardlessof thermal load and environmental conditions. For purposes of thepresent invention the control 17 is provided so as to modify the normaloperation of the expansion valve 13 in a manner to be described. Thecontrol 17 can be a refrigerant system control or a separate valvecontrol.

In order to solve the problem of oil retention in the evaporator asdiscussed hereinabove, the control 17 operates to intermittently, andpreferably in a pulsing manner, substantially increase the refrigerantflow through the evaporator 14 by throttling/unthrottling the expansiondevice 13. That is when the expansion device 13 is periodicallythrottled, pressure is built up in the condenser 12 and pressure isreduced in the evaporator 11. When the expansion device 13 is thenunthrottled or opened, a blast of high pressure refrigerant is forced topass through the expansion device 13 and the evaporator 14. The shortblast of refrigerant will tend to carry the oil that has been trapped inthe evaporator 14 and suction line 15 back to the compressor 11. Suchintermittent blasts of refrigerant will help to return oil that wastrapped in evaporator 11 and suction line 15 and avoid potentialreliability and performance degradation issues.

Referring now to FIG. 1 b, it may be seen that during normal operatingconditions, the discharge pressure at the compressor 11 is at a constantlevel as shown at PD₁. However, when the control 17 operates theexpansion valve 13 in the manner described hereinabove to provide ashort blast (or a series of short blasts) of refrigerant, the dischargepressure at the compressor 11 is substantially and intermittentlyincreased to a level of PD₂ as indicated by the two peaks in FIG. 1 b.It should be noted that the suction pressure at the evaporator andcompressor will be decreasing in unison with the discharge pressurerise, since most of the refrigerant will be intermittently pumped out toa high pressure side. Also, since the oil return operational sequence isexecuted relatively fast, refrigerant system thermal inertia providessufficient cushion so that the refrigerant system performance is notaffected.

Referring now to FIG. 2 a, an alternative embodiment 100 of the presentinvention is shown to include a control 18 for controlling the suctionmodulation valve 16 in a similar manner as described hereinabove. Thesuction modulation valve is positioned on the suction line 15 and istypically utilized to provide part-load operation of a refrigerantsystem. The suction modulation valve 16 may be utilized for oil returnseparately or in conjunction with the expansion valve 13. Furthermore,the individual use of the suction modulation valve 16 may take placewhen an expansion device is not electronically controlled. In the lattercase, the expansion device can be, for example, a TXV type or a fixedrestriction type.

In full-load operation, the suction modulation valve 16 is fully openand doesn't appreciably affect refrigerant flow entering the compressor11 and overall operation of the refrigerant system 100. When the thermalload on the refrigerant system 100 decreases, the suction modulationvalve 16, typically controlled by a stepper motor (not shown), graduallycloses, reducing the refrigerant amount delivered to the compressor 11,until delivered system capacity balances thermal load demands. Thiscontrol strategy matches the compressor capacity to the thermal loaddemands and prevents operation with undesirably low evaporatortemperatures leading to frost formation conditions.

For purposes of the present invention, the control 18 is used tointermittently increase the refrigerant flow through the evaporator 14in a manner similar as described hereinabove. That is, by periodicallythrottling the suction modulation valve 16, pressure is built up in theevaporator 14. When the suction modulation valve 16 is then unthrottledor opened, a short blast of refrigerant will then pass through theevaporator 14 and will carry the oil that has been trapped in theevaporator 14 back to the compressor 11. Once again, such intermittentblasts of refrigerant will help to return refrigerant that was trappedin the suction line 15 as well.

As the control 18 controls the operation of the suction modulation valve16 as described hereinabove, the suction pressure at the compressor 11is substantially and intermittently changed from the normal operatingpressure as shown PS₁ to the lower pressure PS₂ as shown by the threevalleys in FIG. 2 b. At the same time, the pressure in the evaporator 14will be building up, since most of the refrigerant will beintermittently pumped into the evaporator. Once again, since the oilreturn operational sequence is executed relatively fast, refrigerantsystem thermal inertia provides sufficient cushion so that systemperformance is not affected.

Further, the electronically controlled expansion valve 13 and thesuction modulation valve 16 can be operated in conjunction with eachother. For instance, when the expansion valve 13 is intermittentlyclosed, the suction modulation valve 16 is simultaneously opened, sothat most of the refrigerant is collected on a high pressure side of therefrigerant system in preparation to the next blast for oil return tothe compressor 11. Alternatively, when the expansion valve 13 isintermittently opened, the suction modulation valve 16 is simultaneouslyclosed, so that most of the refrigerant is accumulated in the evaporator14 before the next oil return blast.

In another method, at the operating conditions where oil retention mightbe a problem, the amount of refrigerant mass flow circulating throughthe system can be increased by opening the suction modulation valve 16substantially wider, on an intermittent basis, than is required bythermal load demands at these operating conditions. If the suctionmodulation valve 16 were opened wider; that would result in theincreased refrigerant mass flow passing through the evaporator 14 andsuction line 15. As known, it is easier to return oil to the compressor11 when the mass flow rate and refrigerant velocity throughout therefrigerant system are increased.

Analogously, the electronic expansion valve 13 may be openedsubstantially wider than required by the thermal load demands in theconditioned environment, for a relatively short period of time, to allowhigher refrigerant flow rates through the system and thus providingbetter oil return to the compressor 11. As known, these conditions maycause temporal flooding of the compressor 11. Although compressorflooding is an undesired phenomenon in general, it may help in returningoil to the compressor 11, since most of the oil is trapped in thesuperheating section of the evaporator 14 and in the suction line 15.Therefore, the liquid refrigerant will be dissolved in oil, reducing itsviscosity. Furthermore, the liquid refrigerant will mix with dilutedlower viscosity oil and wash it off the internal surfaces bringing theoil back to the suction port of the compressor 11. It should be pointedout that the latter technique could be employed only for the compressorsthat can withstand temporal flooding conditions, such as scroll andscrew compressor types. Also, if the refrigerant system incorporatesboth the electronic expansion valve 13 and the suction modulation valve16, then it is feasible and beneficial to widely open both of these flowcontrol devices for a short period of time to substantially increaserefrigerant flow rate and promote oil return to the compressor 11.

Shown in FIG. 2 c is a graphic representation of the refrigerant massflow rate M through the evaporator when at least one flow control device(the electronically controlled expansion valve 13 or the suctionmodulation valve 16) is throttled/unthrottled in a manner as describedhereinabove. When the respective flow control device is throttled, therefrigerant mass flow is appreciably decreased from the normal operationlevel (as represented by the horizontal line). On the other hand, whenthe respective flow control device is unthrottled, the refrigerant massflow is substantially increased above the normal operation level, andthen upon the throttling it is then again reduced to below the normaloperation level, as shown. As also shown, the throttling/unthrottlingprocess can be repeated several times, if desired

FIG. 2 d shows the change in the refrigerant mass flow rate M throughthe evaporator when either the suction modulation valve 16 or theelectronic expansion valve 13 (or both of them) is opened widely for ashort period of time, as described hereinabove. The dashed line in FIG.2 d represents a time averaged refrigerant mass flow rate that must bemaintained in order to meet the thermal load demands, or in other words,the refrigerant mass flow rate that would be circulating through therefrigerant system without the implementation of the oil return method.The two crests represent the times in which the flow control device iswidely opened (e.g. on the order of 30 seconds). It should be noted,that the time period over which the respective flow control deviceremains widely open, as shown in FIG. 2 d, could be potentially longerthen the throttling time interval shown in FIG. 2 c, since in the lattercase it is more restricted by the reliability concerns. The horizontalline below the dashed line represents the slightly reduced refrigerantmass flow rate at times when the respective flow control device is latermoved toward the normal operating position. In this regard, it should berecognized that this mass flow rate is slightly below a normal valuerequired by the thermal load demand, in order to obtain the desired timeaveraged mass flow rate as represented by the dashed line.

It should be recognized that in the normal course of operation (i.e.aside from the present invention), both the expansion valve 13 and thesuction modulation valve 16 includes some form of control to selectivelyvary the degree in which the valves are opened. In order to carry outthe present invention, one must simply provide further control so as tocause one or the other of the two devices (or both of them) to operatein the manner as described hereinabove. Since all the control isprovided by the software logic modification, no additional hardware isrequired in order to implement the present invention.

Referring now to FIG. 3, the exemplary process by which the control 17or 18 performs its function is shown. In a block 19, the decision ismade by the control as to whether the oil return function is dependenton certain operational and environmental parameters, or whether there isno provision for sensing these parameters. If the system is of the typein which these parameters cannot be sensed, then the control istransferred to a block 23 and proceeds from there.

If the system does include provisions for sensing various parameters,which would indicate that potential conditions existed whereinsufficient amount of oil would not be returned to the compressor, thenthe control proceeds to a block 21 to sense those parameters anddetermine whether the process of the present invention is required inorder to ensure oil return to the compressor as shown in a block 22.Such sensed parameters may include (but are not limited to) thecompressor suction pressure P_(S), the saturation suction temperatureT_(SS), the compressor suction temperature T_(S), the compressordischarge pressure P_(D), the compressor saturation dischargetemperature T_(SD), the compressor discharge temperature T_(D), theambient temperature T_(AMB), the indoor temperature T_(INDOOR), thecompressor current I_(C), the compressor power draw W_(C), etc. Theseparameters may be used separately or in conjunction with each other. Forinstance, if the suction pressure P_(S) is below a predeterminedthreshold, the determination can be made that the refrigerant mass flowis unacceptably low that may lead to oil retention conditions in theevaporator or in the suction line and potential compressor reliabilityproblems. Analogously, a combination of the compressor suction T_(S) anddischarge temperatures T_(D) may lead to similar conclusions. Theseparameter combinations are purely exemplary, and many other cases can beconstructed as well.

If the sensed parameters indicate that there is no problem with oilreturn to the compressor, then the controller proceeds to a block 24such that the timer is reset for a later execution of the control logic.

If the sensed parameters indicate that an oil return process isrequired, then the process moves to the block 23 wherein the expansionvalve 13 or the suction modulation valve 16 (or a combination of both)is throttled/unthrottled in the manner as described hereinabove. In thisregard, it should be recognized that the timing for each of thethrottling and unthrottling steps, as well as the number of times inwhich the cycle is repeated, may vary depending on the operationalconditions and the type of the refrigerant system. As a generalguideline, the valve could be closed for a period of 1-5 seconds andopened for a period 10-30 seconds, with the cycle being repeated from1-10 times in succession. Alternatively, a method of wide opening of therespective flow control device can be executed, where the flow controldevice typically needs to be cycled only once.

It should also be recognized that either of the EXV or ESM valves do notneed to be fully closed or fully opened in the throttling/unthrottlingstep but may be moved to some intermediate position that would providethe desired result of returning the trapped oil without substantiallydeviating from the normal course of operation.

After the oil return process is completed, the timer is reset in theblock 24, such that after a preselected period of time, which may againbe substantially varied to suit the particular system and application,the control returns to the block 19 to repeat the process. A suggestedtime between these successive oil return processes is 2-5 hours.

It should be noted that if there are other flow control devices presentin the refrigerant system they can be used in a similar manner,individually or in conjunction with other valves, as described above, toachieve similar pressure buildup and intermittent refrigerant blastconditions to assist in oil return to the compressor when required.

1. A method of operating a refrigerant system having a compressor, acondenser, an expansion device, an evaporator and a suction modulationvalve, comprising the steps of: operating the system in a normalconventional mode of operation to provide refrigerant flow through theevaporator at a normal rate determined by a thermal demand on therefrigerant system; and periodically, substantially and intermittentlyincreasing the flow of refrigerant through the evaporator such that theflow rate exceeds the normal flow rate to thereby flush out lubricantthat has been entrained in the evaporator or suction line wherein saidstep of increasing the refrigerant flow is accomplished by firstthrottling one of the expansion device and the suction modulation valveand unthrottling the other of the expansion device and the suctionmodulation valve to temporarily build up pressure in the condenser andthen unthrottling the throttled one of the expansion device and thesuction modulation valve to provide a blast of refrigerant through theevaporator.
 2. The method as set forth in claim 1 wherein said step ofincreasing the refrigerant flow is accomplished by first throttling thesuction modulation valve and unthrottling the expansion device to buildup pressure in the evaporator and then unthrottling the suctionmodulation valve to cause a blast of refrigerant through the evaporator.3. The method as set forth in claim 1 wherein said step of increasingthe refrigerant flow is accomplished by first throttling the expansiondevice and unthrottling the suction modulation valve to build uppressure in the condenser and then unthrottling the expansion device tocause a blast of refrigerant through the evaporator.
 4. The method asset forth in claim 3 wherein the throttling position of the expansiondevice corresponds to a fully closed position.
 5. The method as setforth in claim 3 wherein the unthrottling position of the expansiondevice corresponds to a fully open position.
 6. The method as set forthin claim 2 wherein the throttling position of the suction modulationvalve corresponds to a fully closed position.
 7. The method as set forthin claim 2 wherein the unthrottling position of the suction modulationvalve corresponds to a fully open position.
 8. The method as set forthin claim 1 wherein initiation of an oil return cycle is determined basedon a timer setting.
 9. The method as set forth in claim 1 whereininitiation of an oil return cycle is determined based on refrigerantsystem operational and environmental parameters.
 10. The method as setforth in claim 9 wherein said operational and environmental parametersare selected from the group consisting of a compressor suction pressure,saturation suction temperature, compressor suction temperature,compressor discharge pressure, compressor saturation dischargetemperature, compressor discharge temperature, ambient temperature,indoor temperature, compressor current, compressor power draw.
 11. Themethod as set forth in claim 3 wherein the expansion device is throttledfor a period of 1-5 seconds.
 12. The method as set forth in claim 3wherein the expansion device is unthrottled for a period of 10-30seconds.
 13. The method as set forth in claim 3 wherein said throttlingand unthrottling steps of the expansion device are repeated 1-10times insuccession.
 14. The method as set forth in claim 3 further including thesteps of repeating the oil return process every 2-5 hours.
 15. Themethod as set forth in claim 2 wherein the suction modulation valve isthrottled for a period of 1-5 seconds.
 16. The method as set forth inclaim 2 wherein the suction modulation valve is unthrottled for a periodof 10-30 seconds.
 17. The method as set forth in claim 2 wherein saidthrottling and unthrottling steps are repeated 1-10 times in succession.18. The method as set forth in claim 2 and including the steps ofrepeating the oil return process every 2-5 hours.
 19. A vaporcompression system, comprising: a compressor for receiving refrigerantvapor with lubricant entrained therein and compressing the refrigerantvapor; a condenser for receiving the compressed refrigerant vapor withlubricant entrained therein and condensing at least a portion of therefrigerant vapor; an expansion device for receiving the condensedrefrigerant with lubricant entrained therein and expanding therefrigerant to a lower pressure and temperature; an evaporator forreceiving the refrigerant with the lubricant entrained therein from theexpansion device and passing it to the compressor while retaining aportion of the lubricant; a suction modulation valve disposed in asuction line establishing refrigerant flow communication between theevaporator and the compressor; and a control for causing a periodic,substantial and intermittent increase in the flow of refrigerant throughthe evaporator to flush out lubricant that has entrained therein, saidcontrol operative to provide a blast of refrigerant through theevaporator by first throttling one of the expansion device and thesuction modulation valve and unthrottling the other of the expansiondevice and the suction modulation valve and then unthrottling thethrottled one of the expansion device and the suction modulation valve.20. A method of operating a vapor compression system including acompressor, a condenser, an expansion device and an evaporator, whereina lubricant is entrained within the refrigerant and the refrigerant andlubricant mixture is circulated throughout the system, comprising thestep of: periodically providing a blast of refrigerant through theevaporator by causing a substantial increase in the refrigerant flowrate through the evaporator to remove lubricant that has entrainedtherein by first throttling the expansion device to temporarily build uppressure in the condenser and then unthrottling the expansion device.21. The method as set forth in claim 20 wherein the expansion device isthrottled for a period of 1-5 seconds.
 22. The method as set forth inclaim 20 wherein the expansion device is unthrottled for a period of10-30 seconds.
 23. The method as set forth in claim 20 wherein saidthrottling and unthrottling steps of the expansion device are repeated1-10 times in succession.
 24. The method as set forth in claim 20wherein said expansion device is opened for a period of 20-40 seconds.25. A method of operating a vapor compression system including acompressor, a condenser, an expansion device, an evaporator and asuction modulation valve, wherein a lubricant is entrained within therefrigerant and the refrigerant and lubricant mixture is circulatedthroughout the system, comprising the step of: periodically providing ablast of refrigerant through the evaporator by causing a substantialincrease in the refrigerant flow rate through the evaporator to removelubricant that has entrained therein, wherein said step of increasingthe refrigerant flow is accomplished by one of first throttling thesuction modulation valve to build up pressure in the evaporator and thenunthrottling the suction modulation valve and first throttling theexpansion device to build up pressure in the condenser and thenunthrottling the expansion device.
 26. The method as set forth in claim25 wherein said step of increasing the refrigerant flow is accomplishedby first throttling the suction modulation valve and unthrottling theexpansion device to build up pressure in the evaporator and thenunthrottling the suction modulation valve to cause a blast ofrefrigerant through the evaporator.
 27. The method as set forth in claim25 wherein said step of increasing the refrigerant flow is accomplishedby first throttling the expansion device and unthrottling the suctionmodulation valve to build up pressure in the condenser and thenunthrottling the expansion device to cause a blast of refrigerantthrough the evaporator.
 28. The method as set forth in claim 25 furtherincluding the steps of repeating the oil return process every 2-5 hours.29. The method as set forth in claim 26 wherein the suction modulationvalve is throttled for a period of 1-5 seconds.
 30. The method as setforth in claim 26 wherein said suction modulation valve is unthrottledfor a period of 10-30 seconds.
 31. The method as set forth in claim 25wherein said throttling and unthrottling steps of the suction modulationvalve are repeated 1-10 times in succession.
 32. The method as set forthin claim 25 further including the steps of repeating the oil returnprocess every 2-5 hours.
 33. The method as set forth in claim 25 whereinthe suction modulation valve is opened for a period of 20-40 seconds.